In a typical mine roof drilling and bolting machine there is a boom having a drill box on its free end. The machine operator is situated at a control station at one side of the boom. When a roof bolt is to be inserted, the steel of a roof drill is inserted into a chuck in the drill box and the bolt hole is drilled. Then the drill is removed and the shank of a bolt wrench is inserted into the chuck and the roof bolt is wrenched into place. Usually the drill steel and bolt wrench have an annular flange extending outwardly from near the lower end thereof, which flange rests upon the top of the chuck.
From time to time, mechanisms have been provided for centering the drill steels and some of them (O'Neill et al and Bucelluni) have been power-operated by hydraulic rams. However, in neither of these devices are there antifriction members for engaging the drill steel and, taking into account the forces at play and the abrasive nature of the environmental dust, wear of the drill steel and the centering members which engage them is a problem. One object of this invention is to provide centering arms carrying rollers which engage the drill steel to reduce wear on them and the drill steel and to reduce the power required to drive the steel.
Another problem with the prior art centering devices is that they do not accommodate themselves to even slight angular distortions of the drill steel when the latter is inserted into the chuck. Accordingly, another object of this invention is to mount each of two pairs of drill steel engaging rollers at two corners of a triangular chassis plate which is pivoted at its other corner to the inboard end of a lever arm so that the rollers embrace the steel in an accommodating manner.
Because the machine operator is situated at one side of the drill box, it is difficult for him to see the action of the rollers on the far side of the centering device. They should have ample swinging motions to accommodate themselves to the work, but it is undesirable for these rollers to be too far away from the steel when the rollers are swung out to a dis-engaging funtion, and they should not be too close in when the steel is engaged. To meet these needs, means are provided for limiting the outward and inward swinging movements of one of the roller chassis supporting lever arms.